1. Field of the Invention
The present invention relates to a sheet through type image reading apparatus which reads an image on a document while being transported by an image reading portion which cannot be moved.
2. Related Art
Generally, in a sheet through type image reading apparatus, so-called shading correction is performed as follows. A white roller or a white plate is arranged at a position opposed to an image reading portion in a housing. Then, white color on a surface of the white roller or the white plate is read by the image reading portion so as to correct unevenness of brightness due to characteristics of an optical system and an imaging system such that an image having even brightness is obtained.
However, the surface of the white roller or the like is in contact with a document while being transported so as to be easily soiled. If the white roller or the like is soiled by the contact with the document, white color to be reference is disordered so that accuracy of the shading correction is deteriorated.
In terms of this point, in Japanese Unexamined Patent Publication Nos. 5-319613 and 2002-51194, the following configuration of an image reading apparatus is disclosed. In the image reading apparatus, a white reference portion is formed on an outer circumference of a reading roller opposed to an image reading portion. The white reference portion extends along a main scanning direction and has a shape so as not to be in contact with a document. Then, the white reference portion is read by the image reading portion so that shading correction is performed.
In the image reading apparatus as disclosed in Japanese Unexamined Patent Publication No. 5-319613, a bottom surface of a trench formed on the outer circumference of the reading roller is set to a white reference surface. Therefore, the white reference surface is prevented from making into contact with a document while being transported. On the other hand, in the image reading apparatus as disclosed in Japanese Unexamined Patent Publication No. 2002-51194, a part of the outer circumference of the reading roller is made to be a curved surface having a curvature radius which is larger than that of other parts (a curved surface of which curve is moderate). Then, the curved surface is set to a white reference surface. Therefore, the white reference surface is prevented from making into contact with a document while being transported.
The image reading apparatus of this type includes a guiding cover. The guiding cover can be opened and closed with respect to a housing so as to expose or cover a document transportation path in order to remove a document which gets jammed on the document transportation path between an image reading portion and a reading roller or clean a white reference portion of the reading roller, for example. Further, the reading roller is provided at an inner surface side (document transportation path side) of the guiding cover. A driving force is transmitted from a driving motor at the side of the housing to the reading roller at the side of the guiding cover through a belt or a gear transmission system. When the shading correction is performed, the driving motor is driven so as to rotationally drive the reading roller. Then, the white reference portion of the reading roller is opposed to the image reading portion so as to be read by the image reading portion.
However, in the conventional configuration, the driving motor at the side of the housing and the reading roller at the side of the guiding cover are connected through the belt or the gear transmission system such that a driving force can be transmitted therebetween. Therefore, when the guiding cover is opened for removing paper jam or cleaning the reading roller, even if the driving motor is not driven, the reading roller is rotated in accompanied with the opening operation so that the white reference portion is exposed to an inner surface side of the guiding cover in some case. In such a state, the white reference portion can be seen from the outside and a user or the like can contact with the white reference portion. Accordingly, if the white reference portion is carelessly soiled, a possibility of causing deterioration in the accuracy of the shading correction cannot be prevented.